1. Field of the Invention
The present invention relate to an electromagnetic fuel injection apparatus injecting an optimum fuel amount in accordance with a driving state of an internal combustion engine.
2. Description of the Related Art
There have been fuel injectors wherein an electromagnetic valve controls an injected quantity of fuel by seating on a valve seat and unseating from the valve seat a nozzle needle constituting a valve member.
FIG. 7 is a characteristic chart showing the lift of the nozzle needle with respect to a driving pulse driving a coil in a fuel injector of this kind. After the driving pulse becomes ON, the nozzle needle reaches full lift at a certain time delay To from the start of lifting, and after the driving pulse becomes OFF, the nozzle needle reaches zero lift, i.e. seats on the valve seat, after a certain time delay Tc from the start of closing.
The quantity of fuel injected by this fuel injector is controlled by way of the ON time of the driving pulse. To reduce fuel consumption at times of low load, such as when the engine is idling, it is desirable for the minimum injection quantity to be made as small as possible.
FIG. 8 shows the injection capability of a fuel injector by the relationship between the ON time Tq of the driving pulse and the fuel injection quantity. Since the opening area of a nozzle hole is not constant for the period from when the nozzle needle starts to lift until full lift, when Tq is small and the nozzle needle does not reach full lift, the fuel injection quantity is not linear with respect to the ON time. In this region of nonlinearity, exact control of the fuel injection quantity is very difficult, and there is the problem that injection becomes unstable and engine running does not stabilize.
To obtain linearity even at small fuel injection quantities, it is necessary to raise the opening and closing responsiveness of the electromagnetic valve and shorten the time delays which occur on valve opening and valve closing. Fuel injectors which have a driving circuit incorporating a capacitor for accumulating a charge and passing a large current in order to raise the opening and closing responsiveness of the electromagnetic valve are known, but because these driving circuits are very expensive they make it impossible to reduce the cost of the fuel injection system.
A fuel injector in which two driving circuits each having a solenoid are provided to improve the valve opening responsiveness, as shown in JP-A-6-129323, is also known, but even with this fuel injector, because the responsiveness on valve closing does not improve, it has not always been possible to realize a desired minimum injection quantity.
Another prior document relating to an electromagnetic fuel injector is JP-A-7-239050. In this, technology is disclosed wherein a fuel injector (electromagnetic fluid control valve) for injecting fuel into an internal combustion engine has an opening solenoid and a closing solenoid; currents are passed through the respective solenoids at predetermined opening and closing times of a valve member (opening and closing valve) of the fuel injector; and opening and closing is controlled by attracting forces produced at those times.
However, in the fuel injector of JP-A-7-239050, as a result of a spring force and an attracting force acting simultaneously during closing of the fuel injection valve, the impact speed of the valve member is high and its operating noise is loud. This also lowers the durability of the valve seat part. To deal with this, it is conceivable to suppress the operating noise by turning off the current to the closing solenoid immediately before the valve of the fuel injector closes; however, with this kind of control there has been the problem that the valve member tends to bounce back open after the fuel injection finishes, and a secondary injection, supplying excess fuel, takes place.
An object of the present invention to provide a fuel injector which is cheap and has a high valve closing responsiveness.
A fuel injector provided by the invention to achieve this object and other objects comprises a first coil for, when energized by an electric current, magnetizing a first fixed core and thereby attracting a moving core integral with a valve member toward a valve opening direction, and a second coil for, when energized by an electric current, magnetizing a second fixed core and thereby attracting the moving core toward a valve closing direction. As a result, even when a driving circuit does not have a capacitor, like as a battery voltage driving circuit, the valve closing responsiveness of the fuel injector is improved.
Also, the energizing of the second coil is started before the end of energizing of the first coil for holding the valve open. Thus, the attracting force toward the valve closing direction acting on the moving core during closing of the valve becomes large, and the valve closing responsiveness improves.
Further, in a valve closing stroke, energizing of the second coil is ended before the valve member seats on the valve seat. Thus, the moving speed of the valve member just before seating decreases, and operating noise generated by the valve member colliding with the valve seat while the valve closes is reduced.